The present invention relates generally to power steering systems of vehicles, and more particularly to a power steering system of an automotive vehicle that operates a power cylinder according to a steering torque output from a steering wheel for boosting the steering torque.
Recent years, there have been disclosed various power steering systems. One such system is disclosed in Japanese Patent Provisional Publication No. 2002-145087 published May 22, 2002 and assigned to the assignee of the present invention. The power steering system of this publication includes a steering shaft connected to a steering wheel, an output shaft connected to the lower end of the steering shaft, a pinion provided at the lower end of the output shaft, a rack engaged with the pinion, a power cylinder associated with the rack, a reversible oil pump for selectively supplying hydraulic fluid to the first hydraulic chamber at the left-side and the second hydraulic chamber at the right-side of the power cylinder via a first passage and a second passage, a bypass passage provided between the first passage and the second passage, and an electromagnetic valve provided in the bypass passage for opening and closing the bypass passage.
When the steering wheel is turned on driving, a detecting mechanism detects a steering torque. The detecting mechanism outputs a signal of closure to the electromagnetic valve via a control circuit, while the reversible oil pump is rotated normally or reversely. In this way, hydraulic fluid is selectively supplied to one hydraulic chamber and passage from the other hydraulic chamber and passage.
Furthermore, the power steering system includes a failure monitoring circuit. When a failure is detected in the power steering system by the failure monitoring circuit, the electromagnetic valve is opened to provide a fail-safe operation.